In an optical communication system, an optical measurement system, and the like, a variable optical attenuator is incorporated that is a device for variably controlling the transmitted optical power. The device is comprised of a magneto-optical element having the Faraday effect, a permanent magnet that applies a fixed magnetic field to the magneto-optical element, and an electromagnet that applies a variable magnetic field to the magneto-optical element. Typically, the electromagnet is of a structure in which a coil is wound around a C-shaped (a shape of a circle having an open portion) magnetic yoke. By inserting the magneto-optical element into the open portion of the C-shaped magnetic yoke and applying electric current to the coil, a desired variable magnetic field is applied to the magneto-optical element.
The direction of a constant magnetic field Hr generated by the permanent magnet is made approximately parallel with the optical axis of the magneto-optical element, the magneto-optical element is magnetically saturated, and the maximal Faraday rotation in an actual use is caused. In other words, the Faraday rotation angle θf0 in the Faraday arrangement of the permanent magnet and the magneto-optical element and the maximal Faraday rotation angle θfMax in an actual use are made to be equal (θf0=θfMax). Next, a magnetic field Hv whose direction is approximately perpendicular to the direction of the magnetic field Hr generated by the permanent magnet is generated by the electromagnet, the magneto-optical element is arranged in the combined magnetic field formed of the magnetic field of the permanent magnet and the magnetic field of the electromagnet, and the magnitude of the magnetic field generated by the electromagnet is varied in accordance with the magnitude of the current that flows in the coil of the electromagnet, whereby the direction of the combined magnetic field is controlled. The polarization direction can be controlled in accordance with the magnitude of the optical-axis-direction component of the combined magnetic field. The direction θc of the combined magnetic field Hc is given by the following equation:θc=tan−1(Hv/Hr)Variation of the variable magnetic field Hv varies the direction θc of the combined magnetic field Hc. The Faraday rotation angle θf is in accordance with the optical-axis-direction component of the combined magnetic field Hc, and thus given by the following equation; therefore, θf can be controlled by varying θc.θf=θf0×cos θcIn other words, by controlling the current that flows in the coil of the electromagnet for generating Hv, θf can be controlled.
The Faraday rotation θf0 is caused through the fixed magnetic field Hr generated by the permanent magnet, and the Faraday rotation θf is obtained in accordance with the direction θc of the combined magnetic field Hc formed of the variable magnetic field Hv generated by the electromagnet and fixed magnetic field Hr generated by the permanent magnet. Because, being a magnetic field generated by a permanent magnet, Hr is constant and not enabled to be zero, large Hv is required to obtain large θc. In order to obtain large Hv, it is necessary to increase the number of windings of the electromagnet coil or to increase current to be applied to the coil; therefore, the size of the electromagnet is enlarged or the driving voltage is increased. Moreover, there has been a problem in that it takes a long time until the change of the direction θc of the combined magnetic field after the driving voltage has been changed, i.e., operating speed is low.
The case of a variable optical attenuator has a structure in which a first light polarizer, a magneto-optical element, and a second light polarize are arranged in that order, along the optical axis, a saturation magnetic field is applied through a permanent magnet to the magneto-optical element, and a variable magnetic field whose direction is different from that of the saturation magnetic field is applied through an electromagnet to the magneto-optical element. Through the permanent magnet and the electromagnet, external magnetic fields are applied in two or more directions to the magneto-optical element, and the direction of magnetization of the magneto-optical element is changed by changing the vector of the combined magnetic field produced by the permanent magnet and the electromagnet, whereby the Faraday rotation angle of light that passes through the magneto-optical element is controlled. For example, Japanese Patent Laid-Open No. 9-061770 discloses a magneto-optical device, as described above, in which a configuration is employed where block-shaped permanent magnets are arranged above and below the light path, as a means for applying a fixed magnetic field. Additionally, there is a configuration in which permanent magnets of ring-shape or the like are arranged along the optical axis, and a fixed magnetic field is applied in parallel with the optical axis.
As described above, since, in a conventional variable optical attenuator, a permanent magnet is utilized to apply a saturation magnetic field, the magneto-optical element is magnetized in a constant direction by the permanent magnet, even when no electric current is supplied to the electromagnet. Accordingly, the size of a magnetic yoke for an electromagnet utilized for the control of magnetization is rendered large, or the driving voltage is made large; therefore, it is difficult to downsize and speed up the variable optical attenuator.
Moreover, because, when no electric current is applied to the coil of the electromagnet, the variable magnetic field generated by the electromagnet becomes substantially zero, the combined magnetic field to be applied to the magneto-optical element consists only of the component generated by the permanent magnet, whereby the Faraday rotation angle returns to the initial condition.
In contrast, in a Faraday rotator utilized as a self-latching optical switch or the like, the Faraday rotation angle does not returns to the initial condition, even after the exciting current for the coil has been cut off; therefore, the condition in the case where the electric current is applied can be maintained. A Faraday rotator having the self-latching function is comprised of a magneto-optical element having the Faraday effect and a magnetic-field applying device that applies a magnetic field to the magneto-optical element; normally, no permanent magnet is utilized, and the magnetic-field applying device consists only of an electromagnet. As the electromagnet, for example, as disclosed in Japanese Patent Laid-Open No. 8-211347, a structure is utilized in which a coil is wound around a C-shaped magnetic yoke. By inserting the magneto-optical element into the open portion of the C-shaped magnetic yoke and supplying electric current to the coil, a magnetic field is applied to the magneto-optical element. Typically, the control is implemented, with the absolute value of the electric current kept constant, through polarity-reverse action of the electric current, accordingly, the number of possible directions for a magnetic field applied to the magneto-optical element is only two, i.e., the positive and negative directions along a single line; therefore, the number of possible states for the Faraday rotation angle is limited to two states.
In addition, in the Faraday rotator, at least one of the magnetic yoke and the magneto-optical element is formed of a semi-hard magnetic material; both of them are magnetized by exciting current; and after the exciting current is cut off, magnetization remains in the semi-hard magnetic material. In the case where the magneto-optical element is formed of a semi-hard magnetic material, magnetization remains in the magneto-optical element itself; however, in the case where the magnetic yoke is formed of a semi-hard magnetic material, a magnetic field generated by residual magnetization of the magnetic yoke is applied to the magneto-optical element. In both cases, the residual magnetic-field vector and the magnetic-field vector in the case where the electric current is applied are different in magnitude, but the same in direction. Accordingly, even after the exciting current has been cut off, the Faraday rotation angle can be maintained in the same condition as that in the case where the exciting current is flowing; however, maintainable are only two conditions that are possible when the electric current is applied, whereby arbitrary condition cannot be maintained.
As described above, in a conventional variable optical attenuator, by controlling the exciting current supplied to the coil of the electromagnet, the magnetization direction of the magneto-optical element is arbitrarily changed, and, in response to the change of the magnetization direction, the Faraday rotation angle can arbitrarily adjusted; however, the Faraday rotation angle cannot be maintained after the exciting current has been cut off. In contrast, in a conventional Faraday rotator having a self-latching function, even after the exciting current has been cut off, the magnetization direction of the magneto-optical element and the Faraday rotation angle can be maintained; however, the number of maintainable conditions is limited to two.